Paraxylene is a useful chemical intermediate that is oxidized to form terephthalic acid, which is a precursor to polyester. Paraxylene (pX) manufacturing units generally comprise three sections in a recycle loop: 1) a pX recovery section for recovering pX from streams comprising all three of the xylene isomers which produces a pX product stream and a pX lean stream; 2) an isomerization section comprising a xylene isomerization catalyst in a reactor that isomerizes the xylenes in the pX lean stream to near equilibrium; and 3) a fractionation section for separating byproducts produced in the reactor and/or present in the fresh feed. The fresh feed to pX units and thus to the pX recovery section generally also comprises ethylbenzene (EB) which is a structural isomer of the xylenes having a molecular formula that is the same as the xylene isomers. It is not practical to remove EB from these streams by distillation because its boiling point is very close to the xylene isomers. Thus, EB must be converted to xylenes or to byproducts that can be easily separated by distillation to prevent its build-up in the loop. There are several types of xylem isomerization catalysts that convert EB by differing routes. Ethylbenzene isomerization type catalysts provide a means for converting ethylbenzene to xylenes and offer high yields of paraxylene. Naphthenes, ring saturated compounds, are intermediates in the conversion of EB to xylenes and are at near equilibrium with the aromatics in the reactor effluent. These naphthene intermediates need to be efficiently recycled back to the reactor in order to prevent excessive xylene loss.
Prior configurations for paraxylene production and naphthene recycle are energy intensive and thus expensive to operate. For example, some prior art processes employ a separate naphthene tower. Thus, there is a need for an energy efficient process of recycling naphthenes in a paraxylene manufacturing process.